The following description includes information that may be useful in understanding the present invention. It is not an admission that any of the information provided herein is prior art or relevant to the presently claimed invention, or that any publication specifically or implicitly referenced is prior art.
Radio and microwave frequencies are widely used in wireless communication. Antennae utilized in receiving and sending such signals are often used in conjunction with a reflector (e.g., a parabolic reflector) that serves to focus electromagnetic energy in the desired spectral range on a feed that is positioned at the focal point of the reflector and is in communication with a receiver or transmitter. Such an arrangement, however, requires repositioning or aiming of the reflector in order to direct it towards different sources.
As an alternative to the use of a reflector, a lens capable of focusing radio frequency (RF) or microwave frequencies can be used. One suitable lens is a Luneburg lens, a spherically (or substantially spherical) symmetrical lens with a refractive index gradient that decreases from the center to the surface of the sphere. Electromagnetic energy traveling through such a lens necessarily takes the path that it can traverse in the least amount of time. In a classical Luneburg lens the gradient of refractive index is selected so that a focal point for electromagnetic energy impinging across a portion of the sphere is located on the opposing surface of the sphere. Some variations of the Luneburg lens are configured to place the focal point slightly beyond the opposing surface of the sphere in order to accommodate certain feed designs (such as a feed horn). The use of a Luneburg lens permits movement changing the direction of observation or transmission by simply moving the feed about the surface of the lens. In some designs, multiple feeds are arranged on or about the lens in order to permit gathering radio or microwave energy from a number of directions simultaneously without the need to move either the lens or the feeds. For example, a multi-beam station based on a single Luneburg lens can cover 120° in azimuth and thus support multiple beams. In a typical installation, a 1.8 meter spherical Luneburg antenna can support 12 beams having a 10° beam width at 10dB separation for frequencies of 1.7 to 2.7 GHz. Increasing capacity beyond this can be accomplished by decreasing the beam width along the azimuth plane, however this restricts the utility of the device. An alternative is to increase the size of the Luneburg lens, however this approach rapidly encounters issues with the manufacturability of large lenses and the practical issues introduced by the size and weight of the larger lens.
One solution to this problem is to provide multiple lenses, where each lens is equipped with a single feed and where individual feeds are oriented towards different directions. In order to minimize space requirements such lens arrays are typically arranged on a plane in a linear fashion. Unfortunately, such an arrangement greatly restricts the relative angles of reception/transmission of adjacent feeds due to intersection of the transmitted or received signal with a portion of an adjacent lens. For example, in a conventional horizontal arrangement beams with a beam orientation of greater than 30° in the azimuth plane will intersect adjacent lenses. Such antenna arrays are also subject to the generation of undesirable grating lobes as a result of rapid decreases in field amplitudes between adjacent lenses.
Thus, there is still a need for a simple and effective device for providing accessible foci for radio and/or microwave frequencies from multiple directions
All publications herein are incorporated by reference to the same extent as if each individual publication or patent application were specifically and individually indicated to be incorporated by reference. Where a definition or use of a term in an incorporated reference is inconsistent or contrary to the definition of that term provided herein, the definition of that term provided herein applies and the definition of that term in the reference does not apply.